Shorthand notation for middleware paths in `middleware.json`

[bajtos]

This is a follow-up for #66.

Can we hide the structure of the component somehow by handling this a bit differently:

loopback/server/middleware/rest

I don't really care what the reference is as long as "server/middleware" isn't repeated in all middleware.json files.

[bajtos]

I am proposing the following solution:

When the middleware name (path) is in the format {module}/{filename}, loopback-boot will resolve the path by trying multiple locations and using the first one that exists:

1. {module}/{filename} - the path as supplied, e.g. loopback/rest
2. {module}/middleware/{filename} - e.g. loopback/middleware/rest
3. {module}/server/middleware/{filename} - e.g. loopback/server/middleware/rest

Values in any other format will bypass this resolution algorithm and they will be used in the original form:

• full path in a module: loopback/server/middleware/rest
• relative path ./middleware/custom, ./custom, ../logger
• absolute path /usr/local/lib/node_modules/compression

@ritch @raymondfeng thoughts?

---

For posterity, I am posting other ideas I had while thinking about this issue.

At the moment, the middleware id has the following parts:

1. module name (loopback)
2. facet name (server)
3. middleware (a convention)
4. middleware file name (rest)

The module name and the middleware file name are required, we can't remove them.

So far I assumed that the facet name is required as well, since in theory there may be other facets than common,client and server. strongloop/loopback#793 was made in that spirit too.

Keeping that in mind, we can only get rid of the constant "middleware", which is IMHO too little improvement to justify the extra implementation complexity.

In the future, users should use slc loopback to add and configure middleware entries in server/middleware.json. Since they won't type to full path manually, long paths should not be a big issue anymore.

Perhaps it does not make sense to scope the middleware to facets and the middleware directory should be located directly in project root, i.e. the path with become loopback/middleware/rest.